The present invention relates to energy dispersive spectrometry (EDS); and more particularly, the present invention relates to the stabilization of electron beam current in an electron beam column instrument for x-ray microanalysis.
The fundamental basis for EDS analysis is found in the theory of atomic structure. The structure of matter consists of chemical units, called molecules which are composed of atoms held together by chemical bonds. An atom consists of protons and neutrons in a nucleus, and electrons in orbit around the nucleus. Atoms of the various elements differ in the number of protons and neutrons in the nucleus, and also in the number of electrons.
The electrons of an atom occupy orbits, or "shells," of discrete radii essentially concentric with the nucleus. These shells constitute a number of principal quantum paths, with electrons in orbits of larger radius having higher energy levels. There may be from one to seven shells, depending upon the particular element. The shells are usually designated by number (i.e. 1 to 7), starting with the innermost shell, which number is called the principal quantum number n. The shells are also labelled with a letter notation as follows: K (n=1), L (n=2), M (n=3), N (n=4), O (n=5), P (n=6), and Q (n=7). The laws of physics limit the number of electrons in each shell to 2 n.sup.2. Thus, there may be two electrons in the K shell, eight in the L shell, eighteen in the M shell, and thirty-two in N shell.
Each shell of an atom is further characterized by a definite minimum energy required to overcome the attraction of an electron to the nucleus and remove the electron from its shell. This minimum energy valve is referred to as the "binding energy" of an electron. Electrons closest to the nucleus require the highest binding energy and are hardest to remove, and electrons in the outermost shell of an atom require the least binding are the easiest to remove.
When an atom is bombarded by electrons having energies of only a few electron volts, the outer, more weakly bound electrons of the atom can make a transition to an unfilled shell and a higher energy state, while the more tightly bound electrons in the inner closed shells remain in their initial states. The outer electron cannot remain in the higher energy state and makes a transition back to its original ground state. Because the difference in energies is very small between the two energy states, only low energy electromagnetic radiation (photons) emission in the optical spectrum is produced.
If the electrons have sufficient energy to remove an inner electron of an atom, a vacancy is created in an inner shell. Once an inner electron has been removed, an electron from some higher energy shell can transfer to the vacant electron site. Attendant such electron transition, there is an emission of high energy electromagnetic radiation (photons), which have an energy equivalent to the energy difference between the shells of the electron transition.
Creation of a vacant electron site within an inner shell of an atom of an element, by imparting sufficient energy to the atom to overcome the binding energy of the electron occupying the site, followed by the immediate transition of an electron from one of the higher energy shells and photon emission, will, statistically, over a large number of such events, yield a characteristic emission spectrum. The emission energy lines of the spectrum will encompass intrinsic energy intensity (i.e. photons/unit time/unit area) ratios reflecting the probabilities for particular electron transitions associated with the particular inner shell vacancy. The spectrum of photon energies produced is referred to as the "characteristic x-ray spectrum."
The electromagnetic radiation spectrum, often scaled in units of frequency, can be scaled from the point of view of quantum theory in units of energy per photon. On an energy spectrum, x-rays photons have an energy E=hv, where h is Planck's constant and v is the frequency of radiation. Typically, x-ray photons will have energies of thousands of electron volts.
By reason of the linear relationship between the square root of the x-ray line frequency (v) and atomic number and the relationship of x-ray energies to the frequency of x-ray emission lines (E=hv), chemical analysis through the examination of the x-ray spectrum generated by electron bombardment can be accomplished. Such analysis is referred to as x-ray energy-dispersive analysis (EDS).
The manner of producing the characteristic x-ray spectrum for elemental analysis can be that of a primary source of electromagnetic radiation (x-ray tubes, x-rays and gamma rays from radioactive materials) or that of a charged particle beam (electrons, protons, alpha particles). The latter manner of producing characteristic x-rays can be further divided into two distinct categories, one being electron beam excitation and the other being charged particle beams produced by radioactive sources or particle accelerators.
Of primary interest relative to the present invention is the category of electron beam excitation, which is typically carried out with a so-called electron column instrument, such as a scanning electron microscope (SEM), electron microprobe (EMP), or transmission electron microscope (TEM). These electron column instruments, while being x-ray energy spectroscopy (XES) systems, are also referred to as microanalysis systems because of their capability of yielding information on a point-by-point specimen analysis basis rather than on a bulk specimen analysis basis.
In performing chemical analysis using a microanalysis system, the process sometimes being referred to as electron probe analysis (EPA), a beam of high energy (2 to 50 kev) electrons in an evacuated column is focused to a diameter of 0.1-1.0 microns at the surface of a specimen, and may also be scanned across the surface of the specimen. Upon impact with the specimen, the energetic electrons create vacancies in the inner shells of the atoms of the specimen and produce characteristic x-ray generation which is detected and the intensity quantized.
A diagram of a representative EDS analyzer for electron probe analysis is shown in FIG. 1. An electron optical system, generally indicated by the reference numeral 10, focuses a beam of electrons 12 onto a specimen 14. Optical system 10 includes an electron gun 16, magnetic condenser lens 18, objective apertures A.sub.1 and A.sub.2, and magnetic objective lens 20. Characteristic x-rays 22 emitted from specimen 14 impinge upon a detector 24, which is typically a lithium drifted silicon [Si(Li)] detector. An electric potential or bias is applied across detector 24.The absorption of the x-rays creates a free charge in detector 24 proportional to the energy of the x-rays, which is swept out by the bias as a charge pulse appearing at detector terminal 26.
The charge pulse on detector terminal 26 is converted to a voltage pulse by the signal processing circuitry, generally indicated by reference numeral 30, for presentation to multichannel analyzer (MCA) 32. The charge pulse is applied to a preamplifier 34 having a charge sensitive stage for integrating the total charge of the pulse and converting it to a voltage signal. Typically, the preamplifier comprises a cooled field effect transistor (FET) in close proximity to the detector. The output from the preamplifier is split into two signal paths.
One signal path is to an amplifier 36 having a high signal-to-noise ratio and long time constants, resulting in a "slow" signal throughput. In amplifier 36, a combination of differentiation and integration circuits shape the voltage pulse output from preamplifier 34 and sets the pulse width. The period of time during which the voltage output of amplifier 36 is above a threshold valve due to an input pulse, and during which time the amplifier is unable to accept pulses, is referred to as "amplifier dead time." The time constant of the pulse shaping circuitry to yield a desired output pulse width is referred to as the "shaping time constant."
Although a wide output pulse width is desirable to achieve enhanced resolution, wide pulses have an undesirable effect on pulse count rate due to "pulse pileup." The term "pulse pileup" refers to an overlapping of pulses in amplifier 36 when the time period between sequential arrivals of x-ray photons at the detector becomes less than the time required by the amplifier to process an input voltage pulse. Pulse pileup has the effects of creating through summation artificial large pulse amplitudes equivalent to detection of a higher energy photon and of creating a continuum in the region of the energy spectrum above the proper energy location, which reduces the ability to make accurate element analysis in the energy region. Because photons can arrive in any random fashion, input voltage pulses may overlap in any proportion. Thus, the effects of pulse pileup will be variously manifested during an analysis. In either case, however, pulse pileup effects may be referred to as "distortion."
The problem of pulse pileup distortion is overcome by the inclusion in signal processing circuitry 30 of a second pulse processing channel. The other of the two split signal paths from preamplifier 34 is applied to amplifier 38 which functions to process voltage pulses from preamplifier 34 much faster than does amplifier 36. The pulses output by amplifier 38 are applied to discriminator 40 for separation of real pulses from the spectrum of noise passed by amplifier 38. The separation is made based upon a preselected threshold level. The output of discriminator 40 is a normalized rectangular pulse of approximately the same duration as the output pulse from amplifier 38.
The time separation of discriminator output pulses is examined by pileup inspector circuitry 42. If the separation between discriminator pulses indicates that the pulse output from amplifier 36 due to the first input pulse from preamplifier 34 has passed its peak before the second input pulse arrived, the pileup inspector 42 signals pulse rejector 44, in series with amplifier 36, to pass the first amplifier 36 output pulse but reject the second. If the time separation indicates that the second input pulse arrived before the amplifier 36 output pulse had reached its peak, then both pulses are rejected. If, of course, the time separation between preamplifier output pulses is such that no overlap occurs, both pulses are passed by pulse rejector 44. The output pulses from pileup rejector 44 are passed to the multichannel analyzer (MCA) for sorting.
Thus, whereas the first signal processing channel with amplifier 36 functions to process input pulses to yield a high resolution energy spectrum, the second signal processing channel with amplifier 38 and discriminator 40 functions to ascertain the occurrence of input pulses too close in time to be discretely processed by amplifier 36.
MCA 32 performs a sorting function on the basis of pulse amplitude and can be referred to as a pulse height analyzer (PHA). MCA 32 registers the number of pulses that fall within discrete "increments" of pulse height within a range of pulse heights to analyze the x-ray energy spectrum. Typically, the MCA will include an analog-to-digital converter (ADC), and necessarily, a certain amount of time is required in making the conversion. This implies the existence of "dead time" in that the MCA cannot accept pulses for processing.
In view of the existence of dead time for both amplifier 36 and MCA 32, a new term "system dead time" can be defined to denote both "amplifier dead time" and "MCA dead time."
Because quantitative information of an element present in a specimen is contained in the count rate of its characteristic x-ray energy lines over a fixed time, system dead time violates the concept. Thus, there develops the necessity of correcting for dead time. The solution adopted in most EDS systems is that of establishing an "analysis time" corresponding to system "live time," that is, the actual time spent by the system in collecting data. "Analysis time" is distinguished from "acquisition time" which denotes the total time (live time plus dead time) required to complete data collection.
A fixed analysis time is established in the EDS system of FIG. 1 by a live time clock derived from a real time clock gated by clock gate 46. The gate inhibit input to clock gate 46 is generated by dead time control 48, receiving as inputs amplifier busy (AMP BUSY) and MCA bush (MCA BUSY) signals. Dead time control 48 is functionally an OR logic circuit. When amplifier 36 is occupied processing a pulse and is unavailable, AMP BUSY assumes a "high" condition and clock gate 46 inhibits passage of real time clock pulses as live time clock. Similarly, when MCA 32 is doing a conversion, MCA BUSY assumes a "high" condition and passage of real time clock pulses is inhibited.
When x-ray energy intensities are recorded over a fixed analysis time, it is assumed that the electron flux on the specimen, or beam current, which produces the x-rays remains uniform throughout that time. However, the electron beam current is subject to drift, and if the beam current changes during the analysis time, an erroneous count of energies is obtained. Previous approaches to obviating the problem of electron beam current drift have involved the utilization of apparatus for stabilizing the beam current and the application of appropriate corrections to the acquired x-ray spectral data.
Stabilization of electron beam current has been attempted using an aperture A.sub.2 (see FIG. 1) which stops an annular section of the electron flux passing through the condenser lens 18 and provides a current sample proportional to the total electron flux. The aperture current sample provides a means of monitoring beam current stability and can be used as an input signal for a beam current control feedback circuit that regulates the electron gun or as an input to a beam current monitor output device for beam normalization.
Also, the aperture current sample can be converted to a train of pulses and applied as the time basis for elapsed time counting of analysis time. In such case, the aperture current controls analysis time counting instead of a real time clock.
A Faraday cup can also be used to obtain a precise measurement of the electron beam current. The Faraday cup is a closed container having a small hole through which the focused electron beam enters and on which a charge is developed. The charge on the cup is integrated to yield a current equal to beam current, which can be applied to a meter. The need to collect the entire electron beam in the cup for accurate measurement precludes beam monitoring while the specimen is actually being analyzed and requires that specimen analysis be interrupted to measure beam current.
It is also known to compensate for beam current instability by applying an aperture current sample to a current to frequency converter, the output of which is input to an updating scaler count device. The scaler counts achieved, which are related to beam current level, can be stored along with the acquired spectral data. A computer can then be used to normalize the spectral data based on the associated scaler counts. Also, the scaler counts can be read into a computer and a correction factor computer to normalize data as it is being collected.